1. Field of the Invention
The present invention relates to a program generation device which generates a program for groove machining by drilling.
2. Description of the Related Art
Machining of a groove shape is generally performed by machining methods as those illustrated in FIGS. 9A to 9C. “Ramping” illustrated in FIG. 9A is a machining method for forming a groove by cutting a workpiece by a tool which is rotating and is tilted with respect to the workpiece. Further, “extension from hole forming to a groove shape” illustrated in FIG. 9B is a machining method in which after holes are formed on a workpiece by helical machining or drilling, a groove is formed from the holes by lateral surface milling or the like. Meanwhile, “continuous drilling” illustrated in FIG. 9C is a machining method in which a boring such as drilling is repeatedly performed along a groove shape and this “continuous drilling” is mainly used for groove machining (rough machining) of a material which is hard to cut. In the case of a material hard to cut, machining in a direction orthogonal to a tool (spindle) axis direction is mechanically hard to be performed and machining chatter easily occurs, so that machining cannot be performed at a high feed rate, thereby requiring a great deal of machining time. Therefore, in the case where a groove is formed on a material hard to cut, the “groove machining by continuous drilling” which is mechanically strong and in which a boring such as drilling in a tool direction, in which a feed rate can be raised, is repeatedly performed along a groove shape is sometimes employed.
As a prior art technique related to the continuous drilling, Japanese Patent Application Laid-Open No. 10-268919, for example, discloses a technique by which a product-shape profile line is divided so as to obtain constant hole pitch even between a straight line part and a curved line part and thus keep a load constant. Further, Japanese Patent Application Laid-Open No. 2013-161111 discloses a technique by which a thrust sectional area is calculated based on a CAD shape and a tool radius and thus an area cut by one thrust machining is made constant so as to achieve reduction in the number of times of the thrust machining and prolonging of a service life of a tool.
Japanese Patent Application Laid-Open No. 05-92347 discloses a technique by which a cutting area for one thrust machining, which is performed along a profile-line shape, is made constant so as to make a cutting amount constant and enable thrust machining to be performed under the same condition every time, realizing speed-up of the machining and prolonging of a service life of a tool. Further, Japanese Patent Application Laid-Open No. 2008-126377 discloses a technique by which an auxiliary line is generated at a profile-line shape inflection point (high cutting load) and machining points are arranged at predetermined intervals along the auxiliary line so as to prevent a cutting load from increasing, in thrust machining along a profile-line shape. Further, Japanese Patent Application Laid-Open No. 2002-361513 discloses a technique in which thrust machining is performed so as to take a removal stock as large as possible along the ZX shape (a shape in the depth direction) in the thrust machining.
In the case where groove machining by drilling is performed, un-machined parts (cusps) by a tool occur as illustrated in FIG. 10. If the height of these un-machined parts (hereinafter referred to as cusp height) is large, machining time and a load on a tool in a finishing are increased. Therefore, when an operator generates a machining program for groove machining by drilling, the operator needs to calculate this cusp height by herself/himself so as to generate a program by which the cusp height is reduced as much as possible. On the other hand, the cusp height can be reduced as the number of times of drilling is increased, but the machining time is increased along with the increase of the number of times of drilling. Therefore, an operator is required to balance this machining time (the number of times of drilling) and the cusp height. This calculation and balancing have been an operation which takes much time and labor of an operator.
Further, if the groove machining by continuous drilling is simply performed from one direction in sequence, a part of a portion, to which a tool cuts in, of a workpiece has already been removed in the previous machining to generate a bias (lateral asymmetry) in a removal stock in machining of the second and following holes, as illustrated in FIG. 11. Therefore, as a drilling tool tends to violently move in machining of the second and following holes, the machining easily becomes unstable, causing remarkable reduction in a service life of the tool. To deal with such situation, it is necessary to lower a feed rate from the second hole so as to achieve stable machining. However, this lowering causes increase of machining time in the whole machining for the second and following holes disadvantageously.
Any of the prior art techniques disclosed in the above-mentioned patent documents does not provide a technical solving method with respect to these problems.